The present invention relates to teletext systems.
Teletext is a video communication arrangement wherein text, sometimes augmented with simple graphics, presents a message which is transmitted in a highly efficient digital form, generally in fundamental units called pages. In the case of teletext, this data is typically communicated during the vertical interval of a television signal.
In European television and Viewdata systems, 625 lines per frame are used, and the display format is 24 text lines per frame of display. In the NTSC system used in the United States, 525 lines per frame are used, with 20 lines of teletext display. Certain problems arise when a European teletext signal is to be displayed on a receiver constructed to NTSC specification. If a European-format teletext or Viewdata signal were presented to a typical NTSC teletext-equipped receiver, some of the rows of text would not be displayed on the screen.
One solution to this problem is to reformat the teletext or Viewdata information. This would include removing and regenerating timing signals at a different rate, which would require substantial logic circuitry and is too expensive for many applications.
It is therefore an object of the present invention to provide a less expensive arrangement for displaying European format teletext or Viewdata information on an NTSC receiver.
Another object is to provide a means for displaying one system teletext type data originally formatted on a different, incompatible system.